Titanium dioxide nucleating agent systems for foamable polymer compositions

ABSTRACT

The invention concerns nucleating agent systems for foamable polymer compositions, preferably fluoropolymer resins, having foam cell nucleation sites using titanium dioxide based nucleating agents, and foamable polymers comprising the nucleating agents. The nucleating agent systems comprise TiO 2  , certain inorganic salts, and, optionally, sulfonic acid salts and/or phosphonic acid salts. The invention also concerns foamable composition concentrates and foamed thermoplastic insulation material comprising fluoropolymer and the nucleating agent systems.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims the benefit of U.S. ProvisionalApplication No. 60/236,991 filed Sep. 29, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates nucleating agent systems for foamablepolymer compositions, preferably fluoropolymer resins, having foam cellnucleation sites using titanium dioxide (“TiO₂”) based nucleatingagents. The nucleating agent systems comprise synergistic amounts TiO₂,certain inorganic salts, and optionally, sulfonic acid salts and/orphosphonic acid salts. The fluoropolymers useful for making the foamablecompositions described herein are organic polymeric materials comprisingat least about 35 weight percent fluorine and having a melt flow rate(“MFR”) of about 1 g/10 min. to about 100 g/10 min. as measuredaccording to ASTM Standard D1238. The nucleating agents comprise TiO₂ inamounts from about 50 parts per million (“ppm”) to about 2,000 ppm byweight based on the total weight of the composition, inorganic salt inamounts from about 25 ppm to about 3,000 ppm by weight based on thetotal weight of the composition and, optionally, sulfonic acid saltsand/or phosphonic acid salts in amounts of from 0 to about 3,000 ppm byweight based on the total weight of the composition. The foamedfluoropolymer compositions comprising the nucleating agents addressedherein have good structure and cell size, smaller than can be achievedwith other systems, and have exemplary performance characteristicsparticularly for thin walled foamed insulation for conductive materials.

[0004] 2. The Related Art

[0005] Foamed polymer compositions generally comprise nucleating agentsto form a cell structure during processing. Conventional nucleatingagents are boron nitride (“BN”), calcium carbonate, magnesium dioxide,lead oxide, barium oxide, antimony oxide, magnesium carbonate, zinccarbonate, barium carbonate, carbon black, graphite, alumina, calciumsilicate, calcium metasilicate and calcium sulfate.Polytetrafluoroethylene (“PTFE”) is described in U.S. Pat. No. 4,304,713as a nucleating agent for making dielectric compositions for use incoaxial cables, and U.S. Pat. No. 5,314,925 discusses fluoropolyolefinnucleating agents for molded thermoplastics. U.S. Pat. No. 5,716,665addresses the use of PTFE, as well as BN, silicon nitride, alumina, talcand zinc sulfide, as nucleating agents for foamable solid compositionsbased on thermoprocessible perfluoropolymers.

[0006] Despite the number of nucleating agents disclosed in the art, BNhas traditionally been the most preferred, particularly for makingfoamed insulation for conductive wire or other materials. BN, however,must generally be used in amounts from 0.5% to 2.0% by weight of totalcomposition (5,000 ppm to 20,000 ppm) for effective nucleation offluoropolymers. BN is a rather expensive, ranging in price from about$60 per pound to about $90 per pound. The price of BN is a significantdrawback to the use of BN in all applications, and, accordingly, the artof nucleating agents for foamed polymer compositions is evolving toobtain nucleating agent systems which eliminate or reduce the quantityof BN while maintaining or improving on the performance characteristicsneeded for many applications.

[0007] Conductive wire is often used to transmit electronic signals. Theconductor or conductive material must be protected, or insulated, andthermoplastic coatings in a molten state foamed and formed onto andaround the conductor or conductive material are used for this purpose.The thermoplastic materials are selected on the basis of severalcriteria including high dielectric strength, low dielectric constant,and low dissipation factor. It has previously been found that if thethermoplastic material is foamed as it is applied to wire, thedielectric constant is desirably reduced due to the formation ofnumerous small non-interconnected cells in the foam. U.S. Pat. No.3,072,583 discloses a nucleated foaming process for extrudingperfluorinated polymer foam around transmission wire with a gas blowingagent. Foamed insulation using other thermoplastic materials, especiallypolyolefins, is also known in the art.

[0008] In general, after a conductor or conductive material is insulatedwith a foamed polymer, it is twisted into pairs (twinning), then fourpairs are typically twisted together, and a polymer jacket is added tomake the final cable. Adequate crush force of the insulation material isnecessary so that the insulation material will maintain its structuralintegrity, e.g. foamed cell structure, during twinning and consumer use.Structural integrity can be achieved by minimizing or eliminating theamount of elongated cells in the foamed polymer structure and maximizingthe amount of smaller sized cells. Elongated cells are ellipticallyshaped and are merely voids filled with gas, such as nitrogen, andprovide poor mechanical strength to the foamed polymer. As such, duringtwinning, insulation material with a significant number of elongatedcells has a tendency to crush which results in a final cable with poorelectrical properties. Nucleating agents must be selected to obtainspherical shaped cells which provide the mechanical strength fordurability, particularly during twinning operations, to obtain a finalcable with good mechanical strength and good electrical properties.Thus, foamed polymer compositions require effective nucleating agentsystems in effective amounts to make a foamed polymer with theappropriate structural integrity and performance properties,particularly for use as insulation for conductive materials.

[0009] In general, reduction in foam cell size is desired forthermoplastic resins. In particular, for thin wall foams (e.g. 5-10mils) small voids to maximize the number of bubbles are desired formechanical and electrical properties. This is especially so for smallerwire constructions of interest in the electronics field, so that, forexample, foam cell dimensions will be small with respect to the radialdimension of the thin insulation.

[0010] Void space of foamed polymers is also important with regard tothe capacitance of the foamed insulation material. It is important thatthe conductor with foamed insulation (primary) have a capacitance withlow standard deviation. If the voids in the foamed insulation are notuniform, then capacitance variation can arise along a coated wire.Accordingly, small uniform bubbled distribution within the insulationmaterial, which arises by creation of spherical shaped cells, willresult in uniform capacitance. Thus, the nucleating agents are selectedto obtain the uniform cell distribution within the foamed insulationmaterial for stable capacitance.

[0011] Foamed fluoropolymers are also useful in applications other thanwire insulation. Examples of foam structures include foamed sheet forelectrical insulation, thermal insulation and cushioning, foamed pipeand tubing and injection molded pieces. Structural integrity and theminimization or elimination of elongated cells are also important forthese applications.

[0012] A 0.5 to 2.0 weight percent loading of BN alone (5,000 ppm to20,000 ppm) is typically used to provide foam cell nucleation influoropolymer resins having uniform, small cell structure. U.S. Pat. No.4,764,538 discusses the use of synergistic combinations of BN in amountsof 0.02 to 2.0% by weight, and certain inorganic salts in amounts of 25ppm to 3,000 ppm. The preferred range for BN in this system is 500 ppmto 10,000 ppm, and the minimum BN concentration exemplified in thepatent is 2,500 ppm.

[0013] U.S. Pat. No. 4,877,815 describes a class of sulfonic andphosphonic acids, and salts of the acids, which are said to be effectivenucleating agents for a wide variety of thermoplastic materials. Thepatent discusses the combination of certain sulfonic and phosphonicacids with BN, alone or combined with calcium tetraborate for foamingtetrafluoroethylene/hexafluoropropylene copolymers. U.S. Pat. No.5,610,203 concerns the combination of BN crystals grown to final size,inorganic salt and/or sulfonic or phosphonic acids as nucleating agents.

[0014] TiO₂ is a white inorganic material and has a current market priceof about $1 to about $2 per pound. TiO₂ is described in the art as anucleating agent for foamed polymers, however, through the inventiondescribed herein it is discovered that TiO₂ used in certain quantitiesin conjunction with certain inorganic salts and, optionally, sulfonicacid salts and/or phosphonic acid salts act synergistically to result infoamed fluoropolymer compositions with good cell structure and size thancan be achieved with known systems, including conventional nucleatingsystems comprising TiO₂. TiO₂ is also known to have use as a pigment andin topical medicinal preparations.

[0015] U.S. Pat. No. 4,615,850 discusses foamable polyvinylidenefluoride polymers using a nucleating agent having an average particlesize of less than 2 microns. A myriad of nucleating agents arereferenced in the patent, including TiO₂, however the patent provides nodisclosure of TiO₂ used in conjunction with inorganic salts, sulfonicacid salts or phosphonic acid salts, and only foamed polymercompositions comprising calcium carbonate nucleating agents areexemplified. U.S. Pat. No. 5,696,176 addresses foamable polyestercompositions comprising a nucleating agent of a polyolefin and inorganicmaterials, such as TiO₂, talc, chopped fiberglass, alumina, clay andfumed silica. U.S. Pat. No. 5,710,186 discusses closed cell polymer foamcomprising TiO₂, surface treated with either organic materials ororganic and inorganic materials which particulate is used to prevent orreduce agglomeration and reduce thermal conductivity. Nucleating agentsin coating compositions comprising glass microballoons and pigments suchas TiO₂ and ZnO are discussed in U.S. Pat. No. 4,594,368.

[0016] An object of the invention is nucleating agents, and nucleatingagent systems, which enhance the effective nucleation of polymers,particularly fluoropolymers, and provide exemplary cell structure, whichdo not comprise BN and, thus, have lesser cost than nucleating agentsystems comprising BN while having enhanced nucleating effectiveness.

[0017] It is a further object of the invention to obtain foamed polymercompositions comprising these nucleating agents for use in conductivewire jacketing and insulation, or otherwise.

[0018] These and other objectives are achieved by the nucleating agentsdescribed herein comprising a synergistic combination of TiO₂, inorganicsalts and, optionally, sulfonic acid salts and/or phosphonic acid salts.It was unexpectedly discovered that TiO₂ in the TiO₂ based nucleatingagent systems described herein could be used in concentrationssignificantly less than the BN concentrations in conventional systems,and that nucleating agent systems comprising TiO₂, certain inorganicsalts and, optionally, sulfonic acid salts and/or phosphonic acid saltshad better efficiency and provided smaller cell size at very lowconcentrations than nucleating systems in the art. Thus, a substantialcost saving can be achieved with the nucleating agent systems describedherein comprising TiO₂ and the foamed polymers comprising thesenucleating agent systems.

[0019] In the present Specification, all parts and percentages are byweight/weight unless otherwise specified.

SUMMARY OF THE INVENTION

[0020] The invention pertains to nucleating agent systems comprisingsynergistic amounts of TiO₂, certain inorganic salts and, optionally,sulfonic acid salts and/or phosphonic acid salts. The composition of thenucleating agent system will depend on the specific thermoplastic resin,combination of components and the desired foam structure. Typically,however, the nucleating agents comprise TiO₂ in amounts from about 50ppm to about 2,000 ppm, preferably about 100 ppm to about 1,500 ppm, byweight based on the total weight of the composition, inorganic salt inamounts from about 25 ppm to about 3,000 ppm, preferably about 25 ppm toabout 1,000 ppm, most preferably about 50 ppm to about 500 ppm, byweight based on the total weight of the composition and, optionally,sulfonic acid salt and/or phosphonic acid salt, or combinations thereof,in amounts of from 0 to about 3,000 ppm, preferably about 100 ppm toabout 3,000 ppm, most preferably about 200 ppm to about 1,500 ppm, byweight based on the total weight of the composition.

[0021] A wide variety of inorganic salts can be used. Sodium tetraborate(Na₂B₄O₇) and calcium tetraborate (CaB₄O₇), however, are preferred. Thesulfonic acid salts that are preferred in this invention are those saltsof CF₃CF₂ (CF₂CF₂)_(n)CH₂CH₂SO₃X where X is either H or NH₄ and n=1-10,predominantly n=2-4, preferably the barium salt. A useful sulfonic acidsalt for the invention is ZONYLO®BAS, which is barium salt of ZONYL TBS,both available from E. I. DuPont de Nemours and Company (“DuPont”),Wilmington, Del., USA. Other sulfonic acid salts that can be used arepotassium perfluorooctane sulfonate sold under the trademarkFLUORAD®FC-95, potassium perfluorobutane sulfonate (L-7038) bothavailable from 3M, St. Paul, Minn., USA and perfluoroalkyl sulphonicacid available under the tradename FORAFAC®1176 from Atofina Chemicals,Philadelphia, Pa., USA.

[0022] The preferred thermoplastic resins useful for the invention arepolymers having at least about 35 weight percent fluorine and having amelt flow rate (measured in g/10 minutes) of about 1 to about 100,preferably about 5 to about 35. Preferred polymers includepolyvinylidene fluoride; copolymers of tetrafluoroethylene (“TFE”) andperfluoro(methyl vinyl ether) (“PMVE”); copolymers of TFE, PMVE andperfluoro(propyl vinyl ether) (“PPVE”); ethylene/chlorotrifluoroethylene(“E/CTFE”); copolymers of E/CTFE and butyl acrylate and/orhexafluoroisobutylene (“HFIB”) and copolymers of TFE andhexafluoropropylene (“HFP”) optionally containing one or more additionalmonomers, including perfluoro (alkyl vinyl ether) (“PAVE”).

[0023] Thermoplastic resin, TiO₂, inorganic salt, and, optionally, thesulfonic acid salt and/or phosphonic acid salt can be combined inseveral ways to achieve foamable compositions, or concentrates that canbe diluted to foamable compositions. By way of non-limiting examples,thermoplastic resin in powder or cube form, TiO₂, inorganic salt, andsulfonic acid salt and/or phosphonic acid salt if desired, can be dryblended in desired proportions and melt extruded to obtain a compoundedcomposition. Alternatively, individual concentrates of the TiO₂,inorganic salt, and, optionally, sulfonic acid salt and/or phosphonicacid salt in thermoplastic resins can be blended with the same or adifferent compatible thermoplastic resin and then extruded to thedesired composition. Alternatively, one or more of the nucleating agentcomponents in powder form can be combined with concentrates of one ormore of the nucleating agent components, and the thermoplastic resin.Alternatively, one component of the nucleating agent system can first beincorporated into the thermoplastic resin by extrusion, and then thesecond component of the nucleating agent system can be added.

[0024] The foamable thermoplastic resin compositions comprising the TiO₂nucleating agent systems described herein have a variety of uses,including insulating materials for conductors or conductive material,thermal insulation, cushioning, foamed pipe and tubing and moldedarticles, (e.g. injection molded articles). In particular, thenucleating agents can be combined with thermoplastic resin and processedto obtain foamed insulation material for conductors or conductivematerial, the insulation material having a wall thickness of about 0.005to about 0.300 inches, preferably about 0.025 to about 0.200 inches whenthe nucleating agent system does not comprise sulfonic acid salt and/orphosphonic acid salt, and about 0.005 inches to about 0.025 inches whensulfonic acid salt and/or phosphonic acid salt is included.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The nucleating agent systems of the invention comprise TiO₂,certain inorganic salts and, optionally, sulfonic acid salts and/orphosphonic acid salts. The nucleating agents are, generally, used inconjunction with a thermoplastic polymer to make foamed polymerarticles, such as foamed insulation for conductors or conductivematerials. The components of the nucleating agent systems functionsynergistically to provide for foamed polymer compositions with smallercell size and better structure compared to nucleating agents known inthe art, such as those comprising BN. It is discovered that TiO₂ can beused in the nucleating agent system at a range of about 50 ppm to about2,000 ppm to obtain foamed polymer articles having enhanced andacceptable performance characteristics. Thus, the invention allows theuse of TiO₂ in smaller amounts compared to BN, such as grades SHP-325 orCTF-5 available from Carborundum Boron Nitride, Amherst, N.Y., USA,which when used alone requires loading requirements in the range ofabout 5,000 ppm to about 20,000 ppm, and provide foamed polymer articleswith smaller cell size than can be achieved with nucleating systemsknown in the art.

[0026] TiO₂ is a white inorganic powder that is found in nature, and canbe prepared by methods such as the direct combination of titanium andoxygen, treatment of titanium salts or by chemical reaction orhydrolysis. Useful forms of TiO₂ for the nucleating agent systems of theinvention are available from DuPont under the trademark TI-PURE® GradesR-900, R-902, R-960, and R-931. TiO₂ products available from IshiharaCorporation, San Francisco, Calif., USA under the trademark TIPAQUE® andTiO₂ products under the trade names KIMERIA TiO₂ and/or TRONOX availablefrom Kerr-McGee Chemical LLC, Oklahoma City, Okla., USA are also usefulfor the invention. The TiO₂ products may have a TiO₂ content of about70% to 100%, with the balance of the product being alumina, silica,organic materials, as surface coatings, additives, fillers and/orpigments. TiO₂ having inorganic surface coating such as alumina or acombination of alumina and silica are preferred for the invention.

[0027] The nucleating agent systems comprise TiO₂ as a component of anucleating agent for thermoplastic polymers in combination with certaininorganic salts and, optionally, sulfonic acid salts and/or phosphonicacid salts. The nucleating agents comprise TiO₂ in amounts from about 50ppm to about 2,000 ppm, preferably about 100 ppm to about 1,500 ppm, byweight based on the total weight of the composition, inorganic salt inamounts from about 25 ppm to about 3,000 ppm, preferably about 25 ppm toabout 1,000 ppm, most preferably about 50 ppm to about 500 ppm, byweight based on the total weight of the composition and, optionally,sulfonic acid salts and/or phosphonic acid salts, or combinationsthereof, in amounts of from 0 to about 3,000 ppm by weight based on thetotal weight of the composition. When the composition comprises sulfonicacid salts and/or phosphonic acid salts, however, the preferred amountof such salts is about 100 ppm to about 3,000 ppm, most preferably about200 ppm to about 1,500 ppm, by weight based on the total weight of thecomposition. The term “by weight based on the total weight of thecomposition” means the weight of the components of the nucleating agentsystem plus the weight of the thermoplastic polymer and any additives,fillers, pigments or other matter in the composition.

[0028] Inorganic salts useful for the invention include, those selectedfrom the group consisting of lithium carbonate, calcium carbonate,calcium tetraborate, strontium carbonate, sodium carbonate, sodiumtetraborate, sodium sulfite, potassium tetraborate, potassiumpyrosulfate, potassium sulfate, barium nitrate, aluminum phosphate,sodium fluorosilicate and combinations thereof. The inorganic salts thatare preferred for use in the invention are sodium tetraborate andcalcium tetraborate.

[0029] The sulfonic acid salts that can be used in the invention aremetal salts or other salts, such as hydrogen, ammonium, substitutedammonium and quaternary ammonium, of CF₃CF₂(CF₂CF₂)_(n) CH₂CH₂SO₃X whereX is either H or NH₄ and n=1-10, predominantly n=2-4. Barium salts ofsulfonic acid, such as ZONYL BAS from DuPont, are the preferred sulfonicacid salts, however, other metal salts that can be used in the inventionare zirconium, cerium, aluminum, chromium, iron, zinc, calcium,strontium, potassium, or lithium. Other sulfonic acid salts that can beused are potassium perfluorooctane sulfonate, such as FLUORAD FC-95, andpotassium perfluorobutane sulfonate (L-7038), both available from 3M,and FORAFAC 1176 available from Atofina Chemicals.

[0030] Phosphonic acid salts can be used in the TiO₂ nucleating agentsystems, in addition to or in place of the sulfonic acid salts. Bariumsalts of phosphonic acid, such as barium salt of Ashahi Guard AG-530Navailable from Asahi Glass Co., Ltd., Tokyo, Japan are the preferredphosphonic acid salts used in the nucleating agent system, however,phosphonic acid salts of zirconium, cerium, aluminum, chromium, iron,zinc, calcium, strontium, potassium or lithium can also be used.

[0031] Commercially available barium acid salts, such as ZONYL BASavailable from DuPont, can be used in the TiO₂ nucleating agent systems.The barium acid salts of sulfonic acid can also be made using rawmaterial anhydrous BaCl₂ and water borne sulfonic acid, such as ZONYLTBS available from DuPont, and FORAFAC®1033 D and 1187 available fromAtofina Chemicals, according to the following bench formula and processwhich, as will be understood by one skilled in the art, can be scaled-upto make larger batch sizes. For clarity, the process is described withrespect to ZONYL TBS, however, other raw materials can be readily usedin the process. A reaction mass is formed using ZONYL TBS and anhydrousBaCl₂. The ZONYL TBS solution having approximately 30% solids is heatedto approximately 70° C. to achieve a uniform mixture. Then, 50.0 gramsof the uniform mixture of ZONYL TBS is diluted with 200 grams ofde-ionized water in 1 liter flask. After the dilution, 6.63 grams ofanhydrous BaCl₂ dissolved in approximately 20 grams of de-ionized wateris added under constant agitation. After 30 minutes, the reaction massis poured onto a coarse filter paper (P-8) attached to a Boechner funnelto obtain solids which are then washed in a screw top jar by adding 400milliliters of de-ionized water and shaking vigorously. The solids arethen recovered on filter paper and the washing procedure is repeatedtwice more. The solids are then dried for 3 days at 250° C. and thenpassed through a 500 micron Sieve. Subsequently, the dried solids areground in a 4 inch horizontal air mill to obtain the barium salt ofsulfonic acid of the desired size.

[0032] The thermoplastic resins useful in the invention include all suchresins that can be foamed by a gas injection process and/or by use of achemical blowing agent. Preferred resins, include fluoropolymers andpolyolefins. Foamed fluoropolymers are generally foamed by continuousgas injection using nitrogen, carbon dioxide, argon, helium or other gasor gas mixture which is either soluble in or disperses in the moltenresin, using an extruder screw designed for foam operations, and anucleating agent. Nitrogen is especially suited for foaming thefluoropolymers with the TiO₂ nucleating agents because the nitrogeneasily dissolves and/or disperses in molten fluoropolymer resins, andits thermal stability is adequate to survive the holdup time in theextruder when mixed with molten resin at temperatures up to about 450°C.

[0033] A nucleating agent is needed to achieve uniform small-diametercell structure. According to the invention, the nucleating agent systemfor fluoropolymer resins comprising TiO₂, inorganic salts and,optionally, sulfonic acid salts, and/or phosphonic acid salts provide afoamed polymer having smaller cell size than foamed polymers comprisingnucleating agents having BN, including systems where BN is combined withinorganic salts and/or sulfonic acid salts.

[0034] The gas dissolves and/or is dispersed in the molten thermoplasticresin inside the extruder. By adjusting the gas pressure in the feed,the amount of gas dissolved or dispersed in the melt can be controlled.As an alternative process, a chemical blowing agent (a chemical which isunstable at the polymer processing temperature and liberates a gas, suchas nitrogen, carbon dioxide or hydrogen) can be incorporated into thethermoplastic to provide the gas which causes bubble formation.

[0035] Foam cell formation actually starts shortly after the moltenresin containing the blowing agent passes out of the extrusion die. Thegas dissolved or dispersed in the molten resin expands because of thesudden drop in melt pressure as the extrudate exits the extrusion die.The polymer is solidified when the extrudate is cooled, such as beingtreated with water thereby stopping foam cell growth.

[0036] The polymer resins useful in the present invention are organicpolymeric compounds, preferably fluoropolymers comprising at least about35 weight percent fluorine, having a melt flow rate of about 1 g/10 min.to about 100 g/10 min. preferably about 5 g/10 min. to about 35 g/10min. as measured according to ASTM D1238 at the temperature appropriateto each resin.

[0037] Polymers useful for the invention include:

[0038] 1. Homopolymers of chlorotrifluoroethylene (CTFE),2,2-difluoroethylene, or vinylidene fluoride.

[0039] 2. Copolymers of TFE and a monomer selected from the groupconsisting of CTFE, 2,2-difluoroethylene or vinylidene fluoride.

[0040] 3. Copolymers of at least one monomer selected from the groupconsisting of TFE, CTFE, 2-2-difluoroethylene, vinylidene fluoride, andcombinations thereof, and one or more monomers selected from the groupconsisting of hexafluoropropylene (“HFP”), ethylene (“E”), terminallyunsaturated perfluoroolefins having 3-8 carbon atoms, PAVE having 3-8carbon atoms, perfluoro(alkoxy alkyl vinyl ether) having 5-12 carbonatoms, perfluoroalkyl ethylene having 3-8 carbon atoms, and monomers ofthe dioxle family, such as perfluorodioxole, perfluoro (2,2dimethyl-1,3-dioxole), and perfluoro (5 methoxy-1,3-dioxole), andcombinations thereof.

[0041] Preferred fluoropolymers include the copolymers of TFE and PMVE;copolymers of TFE, PMVE and PPVE; copolymers of TFE and PPVE; copolymersof TFE and perfluoro(ethyl vinyl ether); E/CTFE; copolymers of E/CTFEand butyl acrylate and/or HFIB; polyvinylidene fluoride and copolymersof TFE and HFP optionally containing one or more additional monomers,including PAVE.

[0042] Thermoplastic resin, inorganic salt, and, optionally, thesulfonic acid salt and/or phosphonic acid salt can be combined inseveral ways to achieve foamable compositions, or concentrates that canbe diluted to foamable compositions. By way of example and notlimitation, thermoplastic resin in powder or cube form, TiO₂, inorganicsalt, and sulfonic acid salt and/or phosphonic acid salt, if desired,can be dry blended in desired proportions and melt extruded to obtain acompounded composition. Alternatively, individual concentrates of thesame or TiO₂, inorganic salt, or optionally, sulfonic acid salt and/orphosphonic acid salt in thermoplastic resins can be blended withcompatible thermoplastic resin and then extruded to the desiredcomposition if the different thermoplastic resins are compatible.Alternatively, one or more of the nucleating agent components in powderform can be combined with concentrates of one or more of the nucleatingagent components, and the thermoplastic resin. Alternatively, onecomponent of the nucleating agent system can first be incorporated intothe thermoplastic resin by extrusion, and then the second component ofthe nucleating agent system can be added. Other processing methods knowin the art may also be utilized to advantage.

[0043] The concentrations of TiO₂, inorganic salt, and, optionally,sulfonic acid salt and/or phosphonic acid salt to produce an effectivefoam cell nucleation system, that is a system that provides for auniform, small spherical cell structure without any or a significantnumber of elongated cells, will depend on the foam structure beingproduced, the specific resin being used, and the combination ofnucleating agent components. For the foamable thermoplastic resincompositions discussed herein, TiO₂ has concentration in the range ofabout 50 ppm to about 2,000 ppm by weight, based on the total weight ofthe composition, preferably about 100 ppm to about 1,500 ppm. Theconcentration of inorganic salt is in the range of about 25 ppm to about1,000 ppm by weight, based on the total weight of the composition,preferably about 50 ppm to about 500 ppm. Polymers having a very highdielectric constant may require even higher level of inorganic salt,e.g. up to about 3,000 ppm based on the total weight of the composition.The concentration of the sulfonic acid salt and/or phosphonic acid saltis generally 0 to about 3,000 ppm, preferably about 100 ppm to about3,000 ppm, most preferably about 200 ppm to about 1,500 ppm, based onthe total weight of the composition. More than one type of inorganicsalt, sulfonic acid salt and/or phosphonic acid salt can be used, withcombined concentrations as recited above, but generally a single type ineach category selected for the particular foam nucleating agent systemis used. Polymers of the type preferred for the invention generallyrequire from about 5,000 ppm to about 10,000 ppm BN loadings, when usedalone in conventional BN nucleating agent systems, for effectivenucleation.

[0044] When a foamable composition concentrate is prepared, theconcentrations of the components of the foam nucleating agent system areusually some multiple of the concentrations recited above. It isconvenient to make concentrates with concentration of nucleating agent10× to 20× the concentration desired for the foamable extrusioncomposition, in which case one part of the concentrate is blended with 9parts to 19 part of natural resin, i.e., resin containing no nucleatingagent, respectively. The foamable composition concentrate will compriseany of the organic polymers or fluoropolymers discussed herein and fromabout 500 ppm to about 40,000 ppm, preferably about 1,000 ppm to about30,000 ppm titanium dioxide; about 250 ppm to about 60,000 ppm,preferably about 250 ppm to about 20,000 ppm, most preferably about 500ppm to about 10,000 ppm, of the inorganic salts discussed herein and 0to about 60,000 ppm, preferably about 1,000 ppm to about 60,000 ppm,most preferably about 2,000 ppm to about 30,000 ppm, sulfonic acidsalts, phosphonic acid salts and combinations thereof as discussedherein all by weight, based on the total weight of the concentrate, i.e.prior to dilution. While the natural resin should at least be compatiblewith the resin used in the concentrate, it need not have the samechemical composition or be of the same grade. However, it is common touse natural resin of the same grade used as the matrix for theconcentrate.

[0045] The foamable thermoplastic resin compositions of this inventionare useful for making foam structures, especially foamed insulation forconductors or conductive material, the insulation having a wallthickness of about 0.005 inches to about 0.300 inches. The mostpreferable wall thickness for TiO₂/inorganic salt system is about 0.025inches to about 0.200 inches and the most preferable wall thickness forTiO₂/inorganic salt/sulfonic acid salt and/or phosphonic acid saltsystem is about 0.005 inches to about 0.025 inches.

[0046] The invention involves a nucleating agent system comprising TiO₂,inorganic salts, preferably sodium tetraborate or calcium tetraborate,and optionally sulfonic acid salts, preferably barium salts of sulfonicacid, and/or phosphonic acid salts, preferably barium salts ofphosphonic acid. The inventor has discovered that TiO₂, which is arelatively inexpensive material, can be used in effective amounts inconjunction with the inorganic salts and, optionally, sulfonic acidsalts and/or phosphonic acid salts at much lower concentrations than BN,to achieve foamed polymer compositions having better cell structure andperformance properties. A critical range of TiO₂ of between about 50 ppmto about 2,000 ppm, preferably about 100 ppm to about 1,500 ppm, byweight based on the total weight of the composition, used in conjunctionwith the inorganic salt and, optionally, sulfonic acid salt and/orphosphonic acid salt is identified that results in foamed polymerarticles, particularly insulation for conductors and conductivematerials, including thin walled applications. Foam cell size isimportant to the performance properties of the polymer, and the cellsshould not be elongated. The foamed products made with the TiO₂nucleating agent system have unexpectedly better cell size and structurethan foamed polymers made with other agents, including BN. TiO₂ used inconjunction with inorganic salts and, optionally, sulfonic acid saltsand/or phosphonic acid salts at the low concentrations, provides forbetter nucleating, e.g. smaller cell size, than with a correspondingamount of BN and the same salts. Also, substantial cost savings can beachieved in that 1)BN, which costs about $60 to about $90 per poundcompared to $1 to $2 per pound for TiO₂, can be completely eliminatedfrom the foaming process and 2) less amount of TiO₂ is needed than BNfor more effective nucleation.

EXAMPLES

[0047] The foamable fluoropolymer compositions used in the examples wereformulated by developing a fluoropolymer blend concentrate comprisingthe nucleating agent system, and then diluting the concentrate with anatural resin. The foamable fluoropolymer resin concentrates used in thefollowing examples were prepared by combining MFA 6012 resin powder,available from the assignee of invention, Ausimont USA, Inc., Thorofare,N.J., USA under the trademark HYFLON® MFA6012, with additives comprisingthe nucleating agent system, specifically TiO₂, an inorganic salt ofeither sodium tetraborate or calcium tetraborate and, optionally, abarium salt of sulfonic acid or other sulfonic acid salts, as identifiedin Table 1. The barium salt of sulfonic acid was prepared from waterborne ZONYL TBS Fluorosurfactant and BaCl₂ using the bench processdescribed herein. Certain comparative preparations were formulated asnoted.

[0048] The components of the resin concentrate are combined in apolyethylene bag, blended well by shaking and kneaded by hand, and thenextruded through a Model CTSE-V extruder, available from C. W.Brabender, South Hackensack, N.J., USA, equipped with co-rotatingconical twin compounding mixing screws and a strand cutter to pelletizeto form concentrate pellets. The concentrate pellets were then blendedwith pellets of natural fluoropolymer resin and the pellet blend was fedto a standard commercially available wire coating extruder hopper.

[0049] The concentration of additives (e.g. the components of thenucleating agent systems) in the examples are calculated and expressedby weight based on the total weight of the composition, e.g.fluoropolymer and additives. The concentration of the additives weregenerally 20× the concentration in the extrusion composition as setforthin Table 3, so the ratio of natural resin to concentrate was 19/1. Thespecific fluoropolymer resins and additives for foamable extrusion usedin the examples are identified in Table 1, and the concentrations byweight based on the total weight of the composition are set forth inTable 3. Unless otherwise stated, polymer resin available from AusimontUSA, Inc. under the trademark HYFLON MFA 840 with a melt flow rate ofabout 18.3, was employed to let down (dilute) the concentrate to afoamable extrusion composition.

[0050] In the following examples the foaming process was a continuousgas injection foaming process carried out using nitrogen as the blowingagent. High pressure gas injection was used with a single injectionport. A Siebe 45-mm extrusion wire line available from SIEBEEngineering, Industriegebiet Fernthal, Neustadt/Wied, GERMANY was usedin a melt draw extrusion technique. An extruder length/diameter ratio of30/1 was used and the Siebe 45-mm extrusion line was equipped with ascrew designed for foaming operations. Extruder screw design allowed forthe gas injection and had mixing sections to provide a uniform melt.Unless otherwise specified a UF-O crosshead available from Unitek NorthAmerica, Cheshire, Conn., USA was used. Details of the extrusionconditions for the examples are provided in Tables 2 and 3.

[0051] Foam extrudate was characterized as follows. Capacitance anddiameter of the foamed wire insulation were measured and recorded usingthe in-line monitoring devices equipped on the Siebe extrusion line.Void fraction was determined by calculating the density of a 12-inchlength (“l”) of insulation. The void fraction of the insulation wasdetermined by the following method. First the conductor was removed fromthe primary. The average diameter of the conductor (“d₁”) and theaverage diameter of the insulation (“d₂”) was then determined by taking5 readings with a USYS10 bench type micrometer with a rotating sampleholder (USYS 10 H-04A-A DIA FHR-1 Bench Station) available from ZumbachElectronics Corp., Mount Kisco, N.Y., USA. The insulation was thenweighed (“w”). The void fraction was determined using the followingformula:${{Void}\quad {Fraction}} = {1 - \frac{w}{\left\lbrack {\rho \quad l\quad {\pi/4}} \right\rbrack \left\lbrack {d_{2}^{2} - d_{1}^{2}} \right\rbrack}}$

[0052] where ρ is the density of solid polymer and w,l, d₁, and d₂ aredefined above.

[0053] Average foam cell size was determined by taking a photograph of athin cross-section at approximately 50× magnification. The average cellsize reported was calculated by averaging the largest 15 cells. Theresults of the examples are summarized in Table 3. TABLE 1 FluoropolymerResins and Nucleating Agents Code Identification or Description MFA 6012HYFLON MFA 6012 from Ausimont MFA 840 HYFLON MFA 840 from Ausimont NaTBSodium tetraborate CaTB Calcium tetraborate TiO₂ Titanium Dioxide,TI-PURE Grade R900 from DuPont XPH-561 Barium salt of a mixture ofperfluoroalkyl ethane sulfonic acids BN Boron nitride, Grade CTF-5 fromCarborundum Corp.

[0054] TABLE 2 Extruder Details and Extrusion Conditions ExtruderConditions Conditions I Die Diameter (in) 0.185 Tip Diameter (in) 0.095Screw Speed (rpm) 22-23 Melt Cone length (in) 1 Air Gap to water quench(m) 10 Temperatures (° F.) Zone 1 700 Zone 2 735 Zone 3 785 Zone 4 820Clamp 740 Adapter 740 Cross-head 770 Face-Plate 780 Wire Preheat 350

Example 1

[0055] 250 ppm TiO₂ was combined with 100 ppm NaTB to form thenucleating agent system. The results from Example 1 are set forth inTable 3 under Example 1. Surprisingly, the addition of NaTB allows TiO₂to be an efficient nucleating agent at very low concentrations.

Example 2

[0056] To increase the void content of the foamed polymer, the gaspressure was increased from 2,500 psig as used in Example 1 to 3,000psig. 250 ppm TiO₂ was combined with 100 ppm NaTB to form the nucleatingagent system, and the results are set forth in Table 3. For comparisonpurposes, 250 ppm BN and 100 ppm NaTB were tested, and the results ofthis comparative test are set forth in Table 3 as Example 2C. Theresults show that the TiO₂ based nucleating agent system is moreefficient and has a foam cell size 27% smaller than the corresponding BNbased nucleating agent system (Example 2C), in which the same materialswere used with the TiO₂ replaced with BN.

Example 3

[0057] 250 ppm TiO₂, 100 ppm NaTB, and 250 ppm XPH-561, a barium salt ofsulfonic acid, were combined to form the nucleating agent system and theresults are set forth in Table 3 under Example 3. For comparisonpurposes, a combination of 100 ppm NaTB and 250 ppm the XPH-561 bariumsalt of sulfonic acid, without TiO₂, were used as the components of anucleating agent system with the same polymer under the same conditionsto determine the effect of the addition of the TiO₂ to the system. Theresults for the comparative test are set forth in Table 3 as Example 3C.Comparison of the results of Example 3 to Example 3C demonstrate thatusing concentrations of TiO₂ at the low concentration of 250 ppm withthe inorganic salt and sulfonic acid salt produces a foam cell size 24%smaller. Thus, the combination of TiO₂, inorganic salt and sulfonic acidsalt act synergistically to provide more efficient nucleation.

Example 4

[0058] 2,500 ppm TiO₂ were combined with 100 ppm NaTB for the nucleatingagent system. As reflected in Table 3 under Example 4, this amount ofTiO₂ proved to be too efficient and, thus, produced cells that wereelongated. Elongated cells are undesirable, particularly in thin walledapplications, because such cells can cause the foamed insulation to havea low crush force. A good crush force is required since the individualwires are twisted together (twinning). If the crush force is too low thefoamed insulation can get damaged which will produce a poor final cable.Accordingly, TiO₂ concentrations when combined with inorganic saltshould be less than about 2,500 ppm.

Example 5

[0059] 2,500 ppm TiO₂, 100 ppm NaTB, and 250 ppm the XPH-561 barium saltof sulfonic acid were combined and the results are set forth in Table 3under Example 5. Nucleating agent systems comprising 2,500 ppm TiO₂ aretoo efficient at this high loading of TiO₂, as undesirable elongatedcells result. Accordingly, TiO₂ concentrations, even when combined withinorganic salt and sulfonic acid salts, must be less than about 2,500ppm for effective nucleation.

Example 6

[0060] 250 ppm TiO₂ was used alone as the nucleating agent (i.e. noinorganic salts or sulfonic acid salts were used). The results are setforth in Table 3 under Example 6. TiO₂, alone at the low concentrationof 250 ppm does not effectively foam the polymer as only 2.6% voids wereobtained. Thus, in order for TiO₂ to work at the low concentration rangeof the invention, the inorganic salts and, optionally, sulfonic acidsalts are necessary and these salts act synergistically with the TiO₂for effective cell nucleation to obtain foamed polymers with smallercell size compared to systems comprising BN. TABLE 3 Process VariablesNucleating Agents Line XPH- Final Properties N₂ Speed Conductor BN TiO₂NaTB 561 Cell Size Void Diameter Example (psi) (ft/min) (inches) (ppm)(ppm) (ppm) (ppm) (micron) (%) (inches) Comments 1 2500 700 0.0223 — 250100 — 67.2 24.5 0.04057 2 3000 800 0.0223 — 250 100 — 63.4 31.3 0.039822C 3000 700 0.0215 250 — 100 — 80.8 32.1 0.04134 3 3000 700 0.0215 — 250100 250 55.3 26.5 0.04036 3C 3000 700 0.0215 — — 100 250 73.3 33.50.04157 4 2500 700 0.0215 — 2500  100 — 68.1 26.2 0.04045 ElongatedCells 5 2500 700 0.0215 — 2500  100 250 78.0 25.9 0.04027 ElongatedCells 6 2500 700 0.0215 — 250 — — NA  2.6 0.03690 No cell structure

What is claimed is:
 1. A nucleating agent system for foamable polymercompositions comprising about 50 ppm to about 2,000 ppm titanium dioxideby weight based on the total weight of the composition, about 25 ppm toabout 3,000 ppm inorganic salt by weight based on the total weight ofthe composition and up to about 3,000 ppm sulfonic acid salts,phosphonic acid salts or combinations thereof by weight based on thetotal weight of the composition.
 2. The nucleating agent system of claim1 comprising about 100 ppm to about 1,500 ppm titanium dioxide by weightbased on the total weight of the composition.
 3. The nucleating agentsystem of claim 1 comprising from about 25 ppm to about 1,000 ppminorganic salt by weight based on the total weight of the composition.4. The nucleating agent system of claim 1 comprising from about 100 ppmto about 3,000 ppm sulfonic acid salts, phosphonic acid salts orcombinations thereof by weight based on the total weight of thecomposition.
 5. The nucleating agent system of claim 1 wherein thetitanium dioxide has an inorganic surface coating.
 6. The nucleatingagent system of claim 1 wherein the inorganic salts are selected fromthe group consisting of lithium carbonate, calcium carbonate, calciumtetraborate, strontium carbonate, sodium carbonate, sodium tetraborate,sodium sulfite, potassium tetraborate, potassium pyrosulfate, potassiumsulfate, barium nitrate, aluminum phosphate, sodium fluorosilicate andcombinations thereof.
 7. The nucleating agent system of claim 1 whereinthe sulfonic acid salts are metal salts of CF₃CF₂(CF₂CF₂)_(n) CH₂CH₂SO₃Xwhere X is either H or NH₄ and n=1-10.
 8. The nucleating agent system ofclaim 7 wherein the metal is selected from the group consisting ofbarium, zirconium, cerium, aluminum, chromium, iron, zinc, calcium,strontium, potassium, and lithium.
 9. The nucleating agent system ofclaim 1 wherein the sulfonic acid salts are hydrogen, ammonium,substituted ammonium or quaternary ammonium salts ofCF₃CF₂(CF₂CF₂)_(n)CH₂CH₂SO₃X where X is either H or NH₄ and n=1-10. 10.The nucleating agent system of claim 1 wherein the phosphonic acid saltsare salts of barium, zirconium, cerium, aluminum, chromium, iron, zinc,calcium, strontium, potassium or lithium.
 11. A foamable polymercomprising organic polymeric compounds and a nucleating agent systemhaving about 50 ppm to about 2,000 ppm titanium dioxide by weight basedon the total weight of the composition, about 25 ppm to about 3,000 ppminorganic salt by weight based on the total weight of the composition,and up to about 3,000 ppm sulfonic acid salts, phosphonic acid salts orcombinations thereof by weight based on the total weight of thecomposition. 12.A foamable polymer comprising fluoropolymer having atleast about 35 weight percent fluorine and a nucleating agent systemhaving about 50 ppm to about 2,000 ppm titanium dioxide, by weight basedon the total weight of the composition, about 25 ppm to about 3,000 ppminorganic salt, by weight based on the total weight of the composition,and up to about 3,000 ppm sulfonic acid salts, phosphonic acid salts orcombinations thereof, by weight based on the total weight of thecomposition.
 13. The foamable polymer of claim 12 wherein thefluoropolymer has a melt flow rate of about 1 g/10 min. to about 100g/10 min.
 14. The foamable polymer of claim 12 wherein the fluoropolymeris a homopolymer of chlorotrifluoroethylene, 2,2-difluoroethylene, orvinylidene fluoride.
 15. The foamable polymer of claim 12 wherein thefluoropolymer is a copolymer of tetrafluoroethylene and a monomerselected from the group consisting of chlorotrifluoroethylene,2,2-difluoroethylene and vinylidene fluoride.
 16. The foamable polymerof claim 12 wherein the fluoropolymer is a copolymer of at least onemonomer selected from the group consisting of tetrafluoroethylene,chlorotrifluoroethylene, 2-2-difluoroethylene, vinylidene fluoride andcombinations thereof and one or more monomers selected from the groupconsisting of hexafluoropropylene, ethylene, terminally unsaturatedperfluoroolefins having 3-8 carbon atoms, perfluoro (alkyl vinyl ether)having 3-8 carbon atoms, perfluoro(alkoxy alkyl vinyl ether) having 5-12carbon atoms, perfluoroalkyl ethylene having 3-8 carbon atoms, monomersof the dioxle family and combinations thereof.
 17. The foamable polymerof claim 12 wherein the fluoropolymer is selected from the groupconsisting of copolymers of tetrafluoroethylene and perfluoro (methylvinyl ether), copolymers of tetrafluoroethylene, perfluoro (methyl vinylether) and perfluoro (propyl vinyl ether), copolymers oftetrafluoroethylene and perfluro (propyl vinyl ether), copolymers oftetrafluoroethylene and perfluoro (ethyl vinyl ether),ethylene/chlorotrifluoroethylene, copolymers ofethylene/chlorotrifluoroethylene and butyl acrylate, copolymers ofethylene/chlorotrifluoroethylene and hexafluoroisobutylene, copolymersof ethylene/chlorotrifluoroethylene, butyl acrylate andhexafluoroisobutylene and copolymers of tetrafluoro ethylene andhexafluoropropylene optionally containing one or more additionalmonomers.
 18. The foamable polymer of claim 12 wherein the nucleatingsystem has about 100 ppm to about 3,000 ppm sulfonic acid salts,phosphonic acid salts or combinations thereof, by weight based on thetotal weight of the composition.
 19. A foamable composition concentratecomprising fluoropolymer having at least about 35 weight percentfluorine, and a nucleating agent system comprising about 500 ppm toabout 40,000 ppm titanium dioxide by weight based on the total weight ofthe concentrate, about 250 ppm to about 60,000 ppm inorganic salt byweight based on the total weight of the concentrate, and up to about60,000 ppm sulfonic acid salts, phosphonic acid salts or combinationsthereof by weight based on the total weight of the concentrate.
 20. Thefoamable composition concentrate of claim 19 wherein the nucleatingagent system has about 1,000 ppm to about 60,000 ppm sulfonic acidsalts, phosphonic acid salts or combinations thereof by weight based onthe total weight of the concentrate.
 21. The foamable compositionconcentrate of claim 19 wherein the fluoropolymer is a homopolymer ofchlorotrifluoroethylene, 2,2-difluoroethylene, or vinylidene fluoride.22. The foamable composition concentrate of claim 19 wherein thefluoropolymer is a copolymer of tetrafluoroethylene and a monomerselected from the group consisting of chlorotrifluoroethylene,2,2-difluoroethylene and vinylidene fluoride.
 23. The foamablecomposition concentrate of claim 19 wherein the fluoropolymer is acopolymer of at least one monomer selected from the group consisting oftetrafluoroethylene, chlorotrifluoroethylene, 2-2-difluoroethylene,vinylidene fluoride and combinations thereof and one or more monomersselected from the group consisting of hexafluoropropylene, ethylene,terminally unsaturated perfluoroolefins having 3-8 carbon atoms,perfluoro (alkyl vinyl ether) having 3-8 carbon atoms, perfluoro(alkoxyalkyl vinyl ether) having 5-12 carbon atoms, perfluoroalkyl ethylenehaving 3-8 carbon atoms, monomers of the dioxie family and combinationsthereof.
 24. The foamable composition concentrate of claim 19 whereinthe fluoropolymer is selected from the group consisting of copolymers oftetrafluoroethylene and perfluoro (methyl vinyl ether), copolymers oftetrafluoroethylene, perfluoro (methyl vinyl ether) and perfluoro(propyl vinyl ether), copolymers of tetrafluoroethylene and perfluro(propyl vinyl ether), copolymers of tetrafluoroethylene and perfluro(ethyl vinyl ether), ethylene/chlorotrifluoroethylene, copolymers ofethylene/chlorotrifluoroethylene and butyl acrylate, copolymers ofethylene/chlorotrifluoroethylene and hexafluoroisobutylene, copolymersof ethylene/chlorotrifluoroethylene, butyl acrylate andhexafluoroisobutylene and copolymers of tetrafluoro ethylene andhexafluoropropylene optionally containing one or more additionalmonomers
 25. Foamed thermoplastic insulation material comprisingfluoropolymer having at least about 35 weight percent fluorine and anucleating agent system having about 50 ppm to about 2,000 ppm titaniumdioxide by weight based on the total weight of the composition, about 25ppm to about 3,000 ppm inorganic salt by weight based on the totalweight of the composition, and up to about 3,000 ppm sulfonic acidsalts, phosphonic acid salts or combinations thereof by weight based onthe total weight of the composition.
 26. The foamed thermoplasticinsulation material of claim 25 having a wall thickness of about 0.005inches to about 0.300 inches.
 27. The foamed thermoplastic insulationmaterial of claim 25 wherein the nucleating agent system has about 1,000ppm to about 3,000 ppm sulfonic acid salts, phosphonic acid salts orcombinations thereof, by weight based on the total weight of thecomposition.
 28. The foamed thermoplastic insulation material of claim27 wherein the wall thickness is about 0.005 inches to about 0.025inches.